1. Field of the Invention
This invention relates to a particulate magnesium hydroxide useful in imparting flame retardancy to thermoplastic plastics.
2. Description of the Prior Art
Magnesium hydroxide can be produced by adding a water-soluble alkaline material to an aqueous solution of a magnesium salt at atmospheric pressure or slightly above and temperatures from slightly above room temperature to about 100 degrees centigrade or slightly higher. The precipitate which forms on standing forms small crystals whose largest dimension does not exceed about 5 microns and whose thickness is in the range of about 300 to about 900 angstrom units. Representative of prior art processes for the production of magnesium hydroxide are those processes disclosed in U.S. Pat. No. 3,739,058; U.S. Pat. No. 3,508,869; U.S. Pat. No. 2,940,831; and U.S. Pat. No. 3,080,215. A commercial magnesium hydroxide process will produce a particulate material having a size range of about 0.5 to 15 microns, with a median particle size of about 5-6 microns, 20% of the material being smaller than about 3 microns, and 20% being larger than about 9 microns. A typical milling operation might result in a product having a range of 0.1 to 10 microns, with a median particle size of about 1 micron, 20% of the material being smaller than about 0.55 microns, and 20% being larger than about 2.5 microns.
More recently, a magnesium hydroxide which is purported to be particularly suitable for use in combination with thermoplastic synthetic resins to provide flame retardancy has been disclosed in U.S. Pat. No. 4,145,404 and U.S. Pat. No. 4,098,762. Such magnesium hydroxide particles are flat plates typically 0.3-1 micron across with smooth and faceted edges. The thickness of the individual flat plates is typically 500-1500 Angstroms. In addition, Japanese patent disclosure 62-30605, Feb. 9, 1987 to Kinouchi et al. disclose a process for the production of a magnesium hydroxide fine powder utilizing ultrasonic mixing of a water-soluble salt of magnesium and an alkali solution. European Patent Application 0214 494 to Asahi Glass Co. also discloses the preparation of magnesium hydroxide using ultrasonic mixing means.
The use of inorganic fillers such as magnesium hydroxide to provide flame retardancy in thermoplastic resins is disclosed in U.S. Pat. No. 4,067,847; U.S. Pat. No. 4,396,730, and U.S. Pat. No. 4,126,593.
None of these prior art references disclose magnesium hydroxide particles structurally characterized by a predominant fine plate-like particle form having an average particle size of about 0.2 to about 0.4 micron The particles are prepared by a continuous mixing process, as disclosed herein.
The particulate magnesium hydroxide of the invention is particularly suited for use in conjunction with thermoplastic synthetic resins to provide flame retardancy thereto. In this application, the solid particles of magnesium hydroxide can be utilized subsequent to coating with a surfactant or can be utilized without any coating thereon.